The structural properties of the spin crossover compound [Fe(btr)2(NCS)2](H2O), where btr stands for 4,4’-bis-1,2,4-triazole, are investigated by single crystal X-ray diffraction at different temperatures in the thermal spin transition regime. The 104.0(5) K low spin (LS) crystal structure is compared to the room temperature high spin (HS) crystal structure. The C2/c space group is retained in the LS state with an abrupt anisotropic shortening of the b and c cell parameters and a lengthening of a at the transition temperature. The major structural modifications related to the spin transition are a shortening of the Fe-N bond lengths ( $\Delta d_{{\rm Fe}-NCS} = -0.175$ (4) A, $\Delta d_{{\rm Fe-N}(btr)} = -0.213$ (3) A) and a reorientation of the NCS groups with a more linear Fe-N-C-S geometry on going from HS to LS. Diffraction measurements have also been performed at 124 K on a trapped mixed spin state. The observed diffraction pattern shows the coexistence of two crystal lattices corresponding to ordered LS and HS species, which is a direct evidence of spin-like domain formation during the transition. The corresponding fraction of HS species ( $\gamma_{\it HS}\approx 0.10$ ) has been determined by structural refinement using several reference temperature measurements. To investigate dynamical aspects, X-ray data were collected versus time during the spin transition at constant temperature (T = 117.2(2) K). No evidence has been found for any putative presence of an intermediate structural state during the spin transition.